Allow overriding the numbering system during decimal format

[Manishearth]

Fixes #5204

[sffc]

Let's say the data contains:

1. Symbols for en in default numsys latn
2. Symbols for th in default numsys thai
3. Symbols for th in non-default numsys latn

Then I think the behavior should be:

Input Locale	Symbols	Digits	Return value of numbering_system()
en	latn	latn	latn
en-u-nu-thai	latn	thai	thai
th	thai	thai	thai
th-u-nu-latn	latn	latn	latn
en-u-nu-wxyz	latn	latn	latn
th-u-nu-wxyz	thai	thai	thai

I think we should probably fall back gracefully if the numsys is not known? The client can check with the new numbering_system() getter if the digits are what they expected.

[Manishearth]

Makes sense, added fallback. I like the table and I added it to the source as well, this will prevent further confusion on this behavior.

[sffc]

(Please add a test for the table, not just a comment about it)